Lua metatable masking

gistfile1.lua

local t = {}
local mt = { x = 5 }
setmetatable( t, { __index = mt  } ) 

-- No t.x so metatable referenced
print( 1, t.x )   --> 5

-- Add t.x (masking mt.x)
t.x = 4
print( 2, t.x )   --> 4
print( 3, mt.x )  --> 5
t.x = nil

-- Remove t.x (un-masking mt.x)
print( 4, t.x )   --> 5